Single-Phase Induction Motor Design Procedure (Electric Motors)

6.2.8
C. Veinott (1959) presents a method for predicting desired motor performance characteristics, given:
Motor voltage and frequency
Number of poles
Stator punching dimensions
Rotor punching dimensions
Rotor finished dimensions
Stack length
Stacking factor
BH curves for laminations
Main and auxiliary winding data
Rotor squirrel-cage data winding skew
The problem is to predict the performance of a motor built to a proposed design in order to make a decision as to whether the design must be modified to meet the performance specifications.
The approach is to calculate the parameters of the equivalent circuit model plus the losses and compute the performance based on the revolving field theory.
The procedure to accomplish this is as follows:
1. Find the flux per pole from voltage, frequency, and winding data.
2. Find the effective cross sections and lengths of the magnetic circuit (i.e., stator yoke, rotor yoke, stator teeth, rotor teeth, and air gap).
3. From the cross sections, find flux densities in each component.
4. From BH data, find the field intensities in each component; from magnetic pole lengths, compute the mmf required to drive the flux calculated in step 1 through each component; and hence calculate the total mmf required. Calculate the saturation factor as a ratio of total ampere-turns to the air gap ampere-turns.
5. Calculate the resistance of both main and auxiliary windings. The average length of one conductor is obtained by adding the stack length to the average coil throw (ACT) multiplied by an empirical correction factor.
6. Calculate r2, the rotor resistance referred to the main winding. This calculation takes into account the additional length of the rotor bars due to skew and the nonuniform current distribution in the end rings.
7. Calculate the core losses using the flux densities calculated earlier and published loss characteristics for the core material.
8. Obtain friction and windage from a table corresponding to the synchronous speed and frame size.
9. Calculate the leakage reactance x1 and x2. This reactance includes the sum of the slot, end leakage zigzag, belt, and skew reactance; after the total is found, x1 is assumed to be equal to x2.
Compute the slot reactance from the slot constants for both rotor and stator. These constants are a measure of the magnetic permeance between the slot sides per unit length, taking into account the increased mmf applied across the slot as more and more ampere conductors are included as one moves up the slot. Both stator and rotor reactances are included in one calculation.
• Calculate zigzag leakage for the tooth-face dimensions, using a zigzag constant.
• End leakage is a function of the average conductor throw (ACT). Compute this empirically.
• Belt leakage depends on the average number of slots per pole in both rotor and stator and the size of the air gap. Calculate this empirically.
• Calculate skew leakage from the skew factor of the rotor.
Calculate the magnetizing reactance xm from the equation derived from the ratio of the emf induced by the air gap flux to the current required to produce
that field.
The constants for the equivalent circuit model have now been calculated.